The longevity of adults of five genotypes of Tribolium castaneum differing in their body weights was measured at a single constant temperature, 35 degrees C, after they had developed at three constant temperatures, 25, 30 or 35 degrees C, and one alternating temperature 25/35 degrees C (mean = 30 degree C). Two genotypes had been naturally selected for heavy and light body weights, two had been artificially selected for extreme pupal weights and one was the pygmy mutant. The main results are as follows. (1) There is a negative correlation between growth rate and imaginal longevity for four of the five genotypes, when the variations in growth rate are due to the influence of constant developmental temperatures. (2) The genotype has a marked effect on mean longevity and on the slope of the regression of longevity on growth rate. (3) Growth rate is larger and longevity is longer when the larvae are raised at alternating 25/35 degrees C than when they are raised at a constant 30 degrees C. These results seem to confirm the developmental theory of ageing.